The field of optical communications systems using optical fibers, although relatively new, is now well developed with many tens of thousands of optical fibers spanning millions of kilometers installed. An important consideration in the design of such systems is the distance the optical signal can travel through a fiber and still be reliably detected. The optical fibers attenuate the signals, and the first optical communication systems used repeaters to span long distances; signals are detected at the end of one fiber and then regenerated and launched into another fiber. The process is repeated as many times as necessary to span the desired distance. Repeaters, however, require complicated electronic systems to detect and regenerate the signals. A conceptually simpler system periodically amplifies the optical signal without necessarily regenerating it.
Although many types of optical amplifiers have been studied and considered for use in optical communications systems, the most widely used at the present time is the rare earth doped fiber amplifier with erbium being the most commonly used dopant. When optically pumped at suitable power and wavelength, the doped fiber amplifies an incoming signal such as one with a commonly used wavelength such as approximately 1.3 .mu.m or 1.5 .mu.m. Such amplifiers are now well known in the art and need not be described in detail.
The optical amplifier is, of course, subject to design constraints imposed by system considerations. For example, it should have both high gain and low noise for use in a practical system. It should also be easily implemented. Single stage amplifier configurations can not readily met these design constraints with respect to noise and gain. Backward and forward traveling amplified spontaneous emission (ASE) reduces the state of inversion of the fiber. This both decreases the gain and increases the noise figure. In a tandem amplifier, insertion of an isolator between the sections suppresses the backward traveling amplified spontaneous emission (ASE) and avoids degradation of the noise figure, but the gain is still limited. See, for example, Lumholt, IEEE Photonics Technology Letters, 4, pp. 568-570, June 1992. Additionally, in a single stage amplifier, a significant amount of pump power leaves the active fiber and is wasted.
Accordingly, two stage designs using multiple pump lasers have been implemented. Two stage amplifiers provide low noise and gain in the first fiber section; additional gain is provided the second fiber section. Two stage designs suffer from the drawbacks of added complexity and poor efficiency as compared to single stage designs. When both the pump and signal wavelengths are transmitted simultaneously through the isolator, for example, .lambda.hd p=1480 nm and .lambda.hd s=1550 nm, the isolator prevents degradation of amplifier performance by preventing transmission of the backward ASE between the amplifier sections, but it also attenuates both the pump and signal. When both the pump and signal wavelengths can not be simultaneously transmitted through the isolator, for example, .lambda.hd p=980 nm and .lambda.hd s=1550 nm, the pump power is then wasted.
An attempt to improve the two stage design with a single pump has been recently reported. See, for example, Laming et al., IEEE Photonics Technology Letters, 4, pp. 1348-1350 and 1345-1347, December 1992. These papers reported a single pump tandem preamplifier which used an interstage isolator between the second and third multiplexers to achieve both high gain and low noise performance. In this topology, the pump uses a by-pass route while the signal wavelength is transmitted through the isolator. Moreover, the amplifier described requires three multiplexers, increasing the complexity and optical insertion loss in the signal and pump wavelength paths.